The present invention relates in general to a method of and a system for accomplishing the reliable transmission of crucial control messages in CDMA (code division multiple access) communication systems by transmitting these messages at a lower data rate without reducing the transmission power. The increase of the signal to noise ratio of the signal as received by the target device improves the probability of reliable detection without detrimentally altering the interference detected by other receivers in the area
In CDMA systems operating in accordance with the industry agreed upon standard known as IS-95, the call drop and access failure rates are low. However, it is always desirable from a customers standpoint that the call drop rate and access failure rate be zero or at least better than it has been.
When a mobile station (MS) is operating in a cellular system, it attempts to keep a presently controlling BTS (base transceiver station), one that the MS is assigned to, informed of the strength of pilot signals received from nearby base stations through the use of a PSMM (pilot strength measurement message). In accordance with parameters internal to a system, the BTS determines which of the pilot signals listed in the PSMM should be added to an active set of pilots in the MSs memory. This is accomplished through the return of an HDM (Handoff Direction Message) or an EHDM (Extended Handoff Direction Message) sent by the BTS to the MS. To reduce call drops observed in present day CDMA systems, it is crucial that when a pilot needs to be added to the active set of an MS, that the HDM/EHDM so instructing reach the MS successfully. Once the MS successfully receives the returned HDM/EHDM, it can then move the pilot to its active set and start demodulating on the additional or recently detected new pilot.
Although the IS-95 standard provides for (allows) the retransmission of the HDM/EHDM message, it may not be successfully received by the MS due to of many factors environmental and/or physical (such as excessive rf interference or buildings). If the MS fails to receive an HDM/EHDM message, a call in progress could be dropped, since the new pilot may be causing too much interference on the forward link.
It has been observed that a significant percentage of call drops occur as a result of the HDM/EHDM not being successfully received by a MS. A message is classified as being unsuccessfully received if the frame(s) that contained the message are received in error. Similarly, access failures occur in part due to the non-successful receipt of a CAM (channel assignment message). While both of these messages are control messages, the HDM/EHDM is, in accordance with IS-95, transmitted over a traffic channel and the CAM is transmitted over a paging channel.
It is believed by many that there is a need for an increased reliability of receipt of these very crucial messages. One method of increasing the reliability of receipt is to increase the power of the transmitted signal. This however has the detrimental effect of increasing the interference as perceived by other MSs in the vicinity of the target MS. Also, the transmit power on the forward traffic channel cannot be increased indefinitely, since there is a system defined upper limit on it.
It would thus be desirable to have a means for transmitting crucial messages in a more reliable manner without detrimentally affecting other nearby MSs.
The present invention comprises transmitting crucial messages, such as a newly created SHDM (short handoff direction message), in lower rate frames than non-crucial or normal messages while transmitting these crucial messages at the same power level as they would have been transmitted had they been transmitted over full rate frames, to obtain additional processing gain at the receiver.